DJ Hillis, a VR forum member, noticed that on his daily commute his C50
seemed to simply be working too hard (reving too high) at speeds anything
above 65-70 mph indicated. He went looking for taller gears to raise the
gear ratio and drop the rpm. Long story short, he discoverd that with a few
simple modifications he could adapt a complete rear drive unit from a C90
and achieve the ratio he was looking for. This raises the final gear ratio
by 14%. (or drops rpm by that amount, somthing like that). It allows you to stay in each gear longer and just makes the 800 a
more pleasurable ride. You loose a minor amount of bottom end torque, but
barely noticable. I had it on a '05 C50 and really enjoyed it. The largest
benefit is from hiway riding. Here is what the mod entails.
Rear drive units from C90, VL1500 and VS1400 are all the same drive unit.
The C90 and VL1500 have the same size axle hole which is larger than the C50
axle, therefor you have to come up with a bushing that will adapt a C90 or
VL1500 drive to the C50 axle. That is not an issue with a VS1400 drive, for
some reason the 1400 and 800 share the same size axle.
If you were to remove your rear drive unit, you would notice it is mounted
to the swingarm by three mount studs. The circumfrence of the three studs on
a 90/1500 and 1400 drive is slightly larger than the 800 drive. To account
for this the mount holes on the 800 swingarm need to be enlarged to
accomadate the studs of the new drive. This can be done by drilling the
holes oversize (9/16ths for a 90 or VL1500 drive and 1/2 inch for a 1400
drive). Another method to drilling the holes oversize and removing so much
material is to slot the holes with a file. (search this procedure out on the
The next consideration is the drive to wheel hub spacer. The axle spacer
that is on your bike now (I am talking about the one between the drive unit
and the wheel hub) is slightly too long (about 2mm). You can shorten the one
on your bike, but then that makes it unusable if you ever wanted to change
back to you 800 drive (i.e. if you were to sell the bike). You can cut a
spacer from DOM tubing or there are people on the VR who can supply you one
(I'll get to that later).
The other consideration is driveshaft length. Much debate on this topic. DJ
Hillis cuts, sleeves, and welds C90 driveshafts to a length that works. To
my knowledge no one has ever had a problem with a Hillis modified drive.
There is another machinest that I work with in Sturgis SD that takes C50
driveshafts and modifies them for proper length. His process involves some
milling that repositions the shoulder that contacts the u-joint. I've had
both, both work equaly well and are successful
So here is what you need;
A drive unit from a C90, VL1500 or VS1400. For the 90 or 1500 try and get
the driveshaft with it. For a 1400 you would most likly have to get a
modified C50 driveshaft from the guy in Sturgis. You can have the drive
shaft from your C50 modified also.
A bushing to allow the 90 or 1500 drive to accept the 800 axle. Not needed
if using a 1400 drive.
An axle spacer to set the proper space between the drive and wheel hub. I
have made these for people, the machinest in Sturgis (I keep calling him
that 'cause I can't remember his VR name) makes them, and DJ Hillis makes
A driveshaft of correct lenth, be it modified by Hillis or Aric (the guy in
You need a new circa-clip (snapring) and a driveshaft oil seal.
Now the hard part, finding one. DJ Hillis use to purchase take-off drives
from a guy that got them from a place that converted C90's to trikes. That
supply has dried up and Hilllis no longer has a source, however if you can
locate a drive, you can ship it to him (Hillis), he can convert the whole
thing and send you back a drop in kit short of enlarging the holes on your
swingarm. I'm not sure what the cost is these days. The guy he was getting
them from started producing them himself (VR member Hamlings
but it seems
as if that supply has dried up also. Send him a pm an inquire about the
availability. I just don't think they are convering many C90's these days,
but if he has one he can supply you a converted drop in kit, with everything
you need, also. Some people have been successful picking them up on e-bay or
scouring salvage yards.
About the only other thing I can think of at the moment is that the new
drive, once installed will not allow you to use your left side swingarm
(sometimes referred to as the "fake swingarm"). If you got a drive modified
by Aric in Sturgis, the housing of the dive is machined on a CNC machine to
allow you to reinstall the swingarm. A Hillis modified drive allows the same
thing but Hillis accomplishes this by doing some grinding, either way they
both work. Some DIY'ers elect to just leave the "fake" swingarm off and a
few others have been able to grind on the swingarm and make it fit without
touching the drive case.
That's it in a nutshell (if not a rather large nut). I think I'll cut and
paste this to the VR forum as a synopsis of what is involved in doing this
mod. I get asked this alot and it would give me a place I could refer